Laser energy is now in some instances transported over relatively long distances via fiber optic cable. Such laser installations are increasingly used in industrial applications for materials processing, process monitoring, and process control.
As an example, industrial Raman spectroscopy for chemical process monitoring and control uses laser energy from a laser source installed in a central control room instrument. The instrument couples the laser energy into an optical fiber cable that is routed to a remote probe head. The remote probe head is typically installed into a pipeline that may be hundreds of meters away from the laser source.
The fiber cable connecting the remote probe head to the central instrument poses a potential hazard if it is accidentally severed at some point en route. Potential hazards caused by laser energy escaping from a severed fiber optic cable include 1) eye damage to facility personnel, and 2) heating/ignition of explosive gases.
There are many disclosed fiber-optic safety systems, but all present drawbacks in terms of complexity and/or fail-safe operation. U.S. Pat. No. 5,012,087 to Rockstroh et al, entitled FIBER OPTIC SAFETY SYSTEM, detects energy leakage from an optical fiber transmitting a high-power laser beam by providing a second fiber in the same jacket. The second fiber is connected at one end to a light source and at the other end to a photosensitive diode, such that leakage of energy from the power transmitting fiber causes the second fiber to fail, thereby reducing or terminating the transmission of light to the photosensitive diode. Although this system is simple in concept, in the event that the photosensitive diode manages to detect an appropriate amount of light even with failure of the primary fiber, fail-safe operation is not guaranteed. Furthermore, this concept requires electrical power at the remote end of the fiber to power either a light source or a photodetector, which adds the complexity of power cables and, for hazardous environments, the need for an explosion-proof enclosure.
U.S. Pat. No. 5,270,537 to Jacobs, entitled LASER INITIATED ORDINANCE SYSTEM OPTICAL FIBER CONTINUITY TEST, describes a system wherein a primary laser source used to detonate an explosive is switched out of the path of a light-carrying fiber to introduce light from a test source used to detect breaks or discontinuities. The secondary light source generates a series of light pulses which are transmitted through the length of the fiber and reflected by a dichroic coating at the opposite end. A photodetector is positioned near the test light source to detect the reflected pulses, such that if there is a break in the optical fiber, the pulse of light that is reflected will be of lower intensity than would be expected. This system is narrowly tailored for a specific purpose, requiring a large number of optical components and moving parts. This type of testing is directed to applications in which fiber continuity testing is required just prior to launching a single, short pulse of laser light on the fiber. Since the testing is not simultaneous with the transmission of the laser pulse, this approach does not meet the need of continuous monitoring of the continuity of a fiber carrying a cw laser beam.
U.S. Pat. No. 5,729,012 to Wood et al, entitled PHOTOLUMINESCENCE BUILT-IN-TEST FOR OPTICAL SYSTEMS, utilizes the same optical fiber to carry laser energy from a primary source, a test source, and wavelengths emanating and returning from the distal end of an optical fiber. In contrast to the '537 patent to Jacobs, the apparatus of Wood et al includes a photoluminescent material disposed at the junction of an optically-initiated device in the second or distal end of the optical fiber, such that, in a test mode, the photoluminescent material is exposed to light from the test source, resulting in photoluminescence at a third wavelength. The photoluminescent light travels through the optical fiber back to the detector at the source end which, when detected, is used to indicate optical fiber continuity. For applications involving spectroscopic monitoring, the photoluminescent material would produce unwanted spectral signals of its own, which might obscure or mask the desired spectral signals from the sample being monitored. Consequently, the photoluminescent approach is not suitable for applications involving the spectral analysis of various substances.